The instant invention relates to chemical vapor deposition (CVD) apparatus and methods, and more particularly to a method of depositing Parylene AF4 onto the surface of a semiconductor wafer.
Parylene is a general term used to describe a class of poly-p-xylylenes which are derived from a dimer having the structure: ##STR1## wherein X is typically a hydrogen, or a halogen. The most commonly used forms of parylene dimers include the following: ##STR2##
Parylene coatings are obtained from their related dimers by means of a well-known vapor deposition process in which the dimer is vaporized, pyrolized, i.e. cleaved into a monomer vapor form, and fed to a deposition chamber wherein the monomer molecules deposit and polymerize onto a substrate disposed within the deposition chamber. The process occurs according to the following reaction: ##STR3##
Due to their ability to provide thin films and conform to substrates of varied geometric shapes, parylene polymers are ideally suited for use as a conformal external coating in a wide variety of fields, such as for example, in the electronics, automotive, and medical industries.
Octafluoro-[2,2]paracyclophane (Parylene AF4 Dimer) is a fluorine substituted version of the above-noted parylene dimers and has the structure: ##STR4##
It is known that parylene coatings which are derived from the AF4 dimer by the vapor deposition process have a very high melting temperature (about 500.degree. C.) and a very low dielectric constant (about 2.3). These characteristics make Parylene AF4 ideally suited for many high temperature applications, including electronic applications, and potentially as an inter-layer dielectric material in the production of semiconductor chips. The existing parylene coating systems as used with Parylene C, D, and N, typically include a chamber system comprising a vaporization chamber, a pyrolysis chamber coupled to the vaporization chamber, and a deposition chamber coupled to the pyrolysis chamber in which the monomer vapor deposits onto a substrate and polymerizes. The existing coating systems further include a vacuum system coupled to the chambers for creating sub-atmospheric pressure conditions throughout the chamber system.
While the existing parylene deposition systems and know deposition parameters are highly effective in depositing parylene C, D, and N, there are unique process characteristics of the AF4 molecule which prevent the existing parylene coating systems from providing sufficient deposition control, uniformity of layer thickness, material efficiency, and speed of coating to be compatible with existing semiconductor chip manufacturing requirements, semiconductor chip cost structures, and semiconductor chip manufacturing time constraints. Accordingly, there is currently presented a need for a parylene deposition system particularly suited to the deposition of parylene polymers, in particular Parylene AF4, and for a method of depositing Parylene AF4 polymer onto semiconductor wafers.
The instant invention provides a parylene deposition system comprising a vaporization chamber, a pyrolysis chamber, a post-pyrolysis chamber for capturing unpyrolyzed dimer prior to entry into the deposition chamber, a deposition bell having a frusto-conical shape, a filter structure positioned at the deposition chamber inlet, a heated and cooled platen assembly for supporting a semiconductor wafer in the deposition chamber, an electrostatic clamping device for clamping the wafer in intimate thermal contact with the platen, a quartz crystal deposition rate controller, and a vacuum by-pass assembly wherein a high-conductance vacuum outlet is utilized to quickly reduce pressure in the chamber system, and a low-conductance vacuum outlet is utilized to maintain vacuum flow through the deposition chamber during the deposition procedure. The apparatus further includes an atmospheric shroud which completely envelopes the apparatus, and a source of inert gas for flooding the atmospheric shroud with an inert gas.
The vaporization chamber, pyrolysis chamber, post-pyrolysis chamber, and vacuum system, are located within a rectangular housing structure. The platen assembly is preferably located on a top surface of the housing to facilitate placement and removal of the wafers on the platen assembly. The deposition bell is received and secured over the platen assembly to form the deposition chamber.
The platen assembly comprises a thermally conductive platen, a plurality of electric heater elements or hot fluid circulation for heating the platen to a predetermined temperature, and a cooling sub-assembly for cooling the platen to a predetermined temperature. Heating and cooling of a wafer disposed on the platen is accomplished by means of heat conduction through contact with the platen. The cooling sub-assembly comprises a heat exchange coil disposed in intimate thermal contact with the body portion of the platen, and further comprises a heat exchange pump effective for circulating a chilled fluid through the heat exchange coil. In operation, the chilled fluid is circulated through the heat exchange coil to lower the temperature of the wafer on the surface of the platen to a desired temperature. The heating elements are effective for quickly heating the wafer, i.e. raising the temperature of the platen back to room temperature, before removal of the wafer from the deposition chamber.
The electrostatic clamping device for electrostatically clamping the semiconductor wafer is located on the surface of the platen for maintaining the wafer in intimate thermal contact with the platen. Since the wafer temperature is controlled primarily by means of heat conduction from the platen, it is important to maintain an intimate thermal contact of the wafer with the platen surface.
In operation, the wafer is placed onto the platen and clamped in position by the electrostatic chuck which is located on the surface of the platen. The deposition bell is placed over the platen to form a deposition chamber, and thereafter, the atmospheric shroud is flooded with an inert gas to provide an inert atmosphere around the apparatus. A vacuum is then drawn through the entire system while simultaneously cooling the wafer to a desired temperature. The parylene polymer is then deposited onto the wafer according to conventional process steps, i.e. vaporization of the dimer, cleaving of the gaseous dimer into monomer form, and passing the reactive monomer over the surface of the wafer. Prior to removing the wafer from the deposition chamber, the wafer is heated back to room temperature to prevent condensation from forming on the wafer. When desired, the wafer may be further heated to a predetermined annealing temperature to anneal the parylene polymer deposited. The wafer is then cooled back to room temperature and removed from the deposition chamber.
Accordingly, among the objects of the instant invention are: the provision of a parylene deposition apparatus and method effective for the quick and efficient deposition of Parylene AF4 onto silicon wafers in the production of semiconductor chips; the provision of a parylene deposition apparatus including an atmospheric shroud enveloping the apparatus, and a source of inert gas for flooding the shroud during pump down and the actual deposition process; the provision of a parylene deposition apparatus including a heated and cooled platen for supporting the wafer in the deposition chamber and for controlling the temperature of the wafer during deposition procedures; and the further provision of a method for fast, efficient, and cost effective deposition of Parylene Af4 onto the surface of a silicon wafer.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.